1. Field of the Invention
The present invention relates to communication terminals which are connected to a digital network to transmit data from an outgoing call office to an incoming call office, for example, communication terminals which can be connected to an integrated services digital network (hereinafter referred to as ISDN) and control methods thereof. More particularly, the present invention relates to a communication terminal apparatus and its control method which allow the control of notification of an incoming signal to a party to be called and the control of acceptance or rejection of the incoming signal in accordance with predetermined identification data contained in the incoming signal.
2. Description of the Related Art
As communication techniques advance and communication forms diversify various network systems have been developed, one of which is the ISDN.
The ISDN is a digital network which integrally handles various communication processing services such as the transmission of telephone data, information, and facsimile communication data. A communication system using the ISDN has an ISDN exchange which is connected at one side with a digital line switching network, a packet switching network, a common channel signalling network or so on, and which is also connected at the other side with communication terminals installed in homes, such as telephone sets, facsimile machines, voice mail machines and so on, through a subscriber in-home bus having a pair of digital communication network subscriber lines of T (transmission or up) and R (reception or down) lines. In the ISDN-based communication system, a plurality of communication channels are processed on a time-division multiplex basis for each subscriber line and selectively used to realize desired communication between communication terminals. For example, when a fundamental interface has a transmission rate of 192 kbits/sec, two B channels of 64 kbits/sec and one D channel of 16 kbits/sec are multiplexed on a time division basis to perform transmission of data or the like.
In this type of system, the in-home bus connects the communication terminals and an in-home network terminating unit connected to each subscriber line. With the fundamental interface, up to 8 communication terminal apparatuses can be connected to the in-home bus. The user can arbitrarily set a subaddress as terminal identification data at each of these communication terminals. When an incoming signal appears on the in-home bus, each communication terminal searches a subaddress from a call setting message sent from the caller side (ISDN exchange side). If the terminal detects a subaddress, then it collates the detected subaddress with the subaddress of the terminal. Upon a coincidence between both subaddresses, the terminal apparatus performs an operation for the incoming signal. Accordingly, when the caller transmits such a call control message that has not only a called number corresponding to a subscriber line, but also a desired subaddress inserted therein, communication with a desired one of a plurality of communication terminals connected to a single subscriber line can be realized. In this system, if the terminal fails to find a subaddress in an incoming signal, then all the terminals can answer to the incoming signal.
In this case, an interconnection of the originating terminal to another in-home bus through another network terminating unit is established by the ISDN exchange located at a central station.
More specifically, when it is desired to communicate through the ISDN, a calling terminal sends a call setting message, including the telephone number of an in-home bus, through the 16-kbps control signal channel (D channel) and its network terminating unit to the network. On the call receiving side, a plurality of receiver terminals connected to the receiver in-home bus receive the call setting message transmitted from the network to a receiver 16 kbps control signal channel through a receiver network terminating unit and perform ringing operations. When it is desired that the calling terminal selectively call a desired one of the plurality of receiver terminals connected to the receiver in-home bus, the calling terminal sends to the network a call setting message that contains a receiver in-home bus telephone-number element followed by a subaddress, such as a receiver-terminal identification data element. The receiver terminals then check the receiver-terminal identification data element in the call setting message received from the network. If one of the receiver terminals finds its own terminal identification data, then it sends a ringing element back to the network and starts its ringing data operation.
In the above case, it has been possible for the call sender to identify or select a desired one of the call receiver terminals, but it has been impossible that the sender selects an individual person be called at each of the call receiver terminals.
For this reason, it has been common practice that any one of the users commonly using the terminal must once answer the incoming call, talk with the party, ask the desired speaker and finally call him or her to answer it.
In particular, when a single communication terminal is commonly used by a plurality of persons, it would be convenient that an incoming signal could be selectively directed to a particular individual person. Such a prior art communication terminal apparatus as mentioned above, however, has a problem in that identification of the communication terminal is carried out on the basis of its single subaddress, and thus the terminal cannot identify the desired one of a plurality of common users for the terminal apparatus.
In addition, with communication terminals connected to the in-home bus of the digital communication network mentioned above, each of the terminals is provided with a display (for example, an LCD (liquid crystal display)) on which, upon reception of an incoming signal, its caller identification data (for example, telephone number) is indicated. Such a communication terminal apparatus is also known that the apparatus stores caller identification data and each time it receives an incoming signal, it outputs the stored identification data for user's for the user's reference. This terminal apparatus is advantageous in that even when the target user leaves her or his desk, one of the other users can know the telephone number of the calling party.
However, in companies and business offices an inconvenience occurs when a single communication terminal apparatus is commonly used by a plurality of persons, even when the apparatus outputs the identification data of a caller on an incoming signal, because it took much time for every user to look at their own communication terminal.
In addition, the above system is inconvenient because the system is arranged to ring only at terminal having a subaddress coincided with that of an incoming signal. Therefore, the call receiver cannot know that he is being called unless the call receiver troublesomely goes to the ringing terminal.